"Okay, the most important thing you're going to learn today is how to tread water," Luke says, ducking down so he's level with Jack. "If you know how to tread water properly then it doesn't take much energy at all and you could theoretically do it for hours, which is good to know how to do in an emergency situation. I'm going to reach under the water for your legs, okay? And I'll show you how to move them."

Luke ducks a little further, dropping his hands under the water, his fingers curling gently over Jack's ankles.

"Feel that?" he asks, then slowly and carefully begins to move Jack's legs for him. "That's easy, right? All you have to do is that gentle kick and then move your arms back and forth under the water like you're gently scooping it away from you."